Current trends in the supply and prices of petroleum crudes are driving refining processes to use poorer quality crude oils to produce lubricating oil basestocks. High quality lubricating oils must have a high viscosity index (VI), low volatility, good low temperature fluidity and high stability. These requirements, in turn, are being pushed by the requirements of modern engine design which, in its own turn, is being driven by regulatory and economic pressures for ever higher efficiencies at an unprecedented pace. The demands on lube supply are increasingly being met by hydrocracking, the process originally applied predominantly to fuels manufacture but now being applied to lube basestock production with distillate fractions of lower boiling range being produced at the same time as valuable fuel products.
The trends in lube quality are reflected in the increasing proportions of the higher quality lube basestocks coming from the refineries, as measured by the outputs of the various API Groups. The solvent-refined Group I base oils contain less than 90 percent saturates, greater than 0.03 percent sulfur and have a viscosity-index range of 80 to 120. Group II base oils are often manufactured by hydrocracking and are defined as being more than 90 percent saturates, less than 0.03 percent sulfur and with a viscosity index of 80 to 120. Group II base oils have better antioxidation properties, a clearer color and cost more in comparison to Group I base oils. Group III oils contain more than 90 percent saturates, less than 0.03 percent sulfur and have a viscosity index over 120. In comparison to a little more than a decade ago API Group II base oils now constitute up to 47 percent of the capacity of plants compared to 21 percent for both Group II and III base oils a decade ago. Although Group III plant capacity is currently limited, it is expected to rise with time. At the same time Group I base oils previously made up 56 percent of the capacity, compared to 28 percent of the capacity in today's plants.
Conventional methods for producing both fuels and lubricating oil products from a single integrated hydrocracking system are typically optimized for one type of product, with the properties and yield of the second type of product being dictated by conditions imposed on the system. For example, a fuels hydrocracker, operated at high severity for producing fuels, may also produce a lubricating oil product stream from the unconverted high boiling fractions although the range of lubricating oil fractions from a single unit may be limited. In addition, fuels hydrocrackers are typically operated with high recycle ratios in order to increase the yield of the desired fuel products—mainly middle distillates such as road diesel and aviation kerosene—so that the yield of higher boiling lube fractions is limited. The repeated passes through the unit occurring with the high recycle ratios are likely to result in excessive conversion to lower boiling products and may possibly degrade performance indicia as lube basestocks. Accordingly, it would be desirable to develop processes for producing high quality lubricant basestock fractions by the hydrocracking process.
Hydrocracking of vacuum gas oils and heavier feeds to produce lube oil fractions is typically restricted to operation at low conversion in order to produce base stocks over a specific viscosity range. This usually implies that the higher boiling product fractions contain high concentrations of cyclic (naphthenic and even untreated aromatic) components. Removal of the these components by hydroprocessing requires high temperatures and pressures which lead to the consumption of large amounts of hydrogen during the processing as the ring structures are opened and hydrogenated.
Traditionally, two different types of lube products requiring different processing severities for their production have been made in blocked operation in order to preserve optimal lube yields. The blocked operation is done in steps, first one lube grade is produced and then the other. The plant is used to produced one lube grade at a time and for this reason, operating conditions of the reactor(s) can be adjusted to get the highest yield at a given quality. One of the problems of blocked operation is that the feed needs to be segregated for each type of lubricant product so it needs a distillation step and tankage to store the different feeds. In addition, there are transition times between the blocks in which the process needs to reach steady state for production of a consistent uniform product quality.
Another way of making two or more lube grades is to process a wide cut feed in one set of reactors targeting one key specification, then fractionating to produce the two or more lube grades. This process is results in yields which are less than optimal with a give-away in product yields and qualities such as VI, viscosity, cold flow properties.
Various proposals have been made for the production of lube fractions from heavy oils by hydrocracking high boiling fractions. U.S. Pat. No. 5,580,442 (Kwon et al) discloses a process in which a vacuum gas oil (VGO) is hydrotreated to remove impurities and then hydrocracked. The light hydrocarbons created by cracking are then removed by distillation and a lube boiling range fraction is separated from the unconverted bottoms fraction.
U.S. Pat. No. 5,985,132 (Hoehn) discloses a method using a lubes hydrocracker at a low conversion with the a second hydrocracking step to produce fuels.
U.S. Pat. No. 6,623,624 (Cash) discloses a hydrocracking process for producing fuels asserting the flexibility to recover one or more lubricating oil products over a range of viscosities and viscosity indices. The process functions by hydroprocessing the feed at either hydrotreating or hydrocracking conditions to remove impurities and separating the liquid fraction effluent by boiling point range to yield a lubricating oil product as well as a fuel product and a bottom fraction. The bottom fraction is passed to a hydrocracking step to yield a product which can be fractionated into overhead and a recycle stream which is sent back to the hydrocracking for fuels production.
US 2014262941 (Rameseshan) discloses a process for producing multiple grades of lube oil base feedstock in a two-stage hydrocracking unit. Effluent from the first hydrocracking step is sent to a separation zone with a heavy liquid effluent being fractionated off. A portion of the bottom stream from the fractionator is passed to a second hydrocracking step form which the effluent is fractionated to produce a second lube boiling range fraction.